The Restoration of Ecological Interactions: Plant–Pollinator

View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Sussex Research Online Journal of Applied Ecology 2008, 45, 742–752 doi: 10.1111/j.1365-2664.2007.01390.x TheBlackwell Publishing Ltd restoration of ecological interactions: plant–pollinator networks on ancient and restored heathlands Mikael Lytzau Forup†, Kate S. E. Henson, Paul G. Craze and Jane Memmott* School of Biological Sciences, University of Bristol, Woodland Road, Bristol, BS8 1UG, UK Summary 1. Attempts to restore damaged ecosystems usually emphasize structural aspects of biodiversity, such as species richness and abundance. An alternative is to emphasize functional aspects, such as patterns of interaction between species. Pollination is a ubiquitous interaction between plants and animals. Patterns in plant–pollinator interactions can be analysed with a food web or complex- systems approach and comparing pollination webs between restored and reference sites can be used to test whether ecological restoration has taken place. 2. Using an ecological network approach, we compared plant–pollinator interactions on four pairs of restored and ancient heathlands 11 and 14 years following initiation of restoration management. We used the network data to test whether visitation by pollinators had been restored and we calculated pollinator importance indices for each insect species on the eight sites. Finally, we compared the robustness of the restored and ancient networks to species loss. 3. Plant and pollinator communities were established successfully on the restored sites. There was little evidence of movement of pollinators from ancient sites onto adjacent restored sites, although paired sites correlated in pollinator species richness in both years. There was little insect species overlap within each heathland between 2001 and 2004. 4. A few widespread insect species dominated the communities and were the main pollinators. The most important pollinators were typically honeybees (Apis mellifera), species of bumblebee (Bombus spp.) and one hoverfly species (Episyrphus balteatus). The interaction networks were signi- ficantly less complex on restored heathlands, in terms of connectance values, although in 2004 the low values might reflect the negative relationship between connectance and species richness. Finally, there was a trend of restored networks being more susceptible to perturbation than ancient networks, although this needs to be interpreted with caution. 5. Synthesis and applications. Ecological networks provide a powerful tool for assessing the outcome of restoration programmes. Our results indicate that heathland restoration does not have to occur immediately adjacent to ancient heathland for functional pollinator communities to be established. Moreover, in terms of restoring pollinator interactions, heathland managers need only be concerned with the most common insect species. Our focus on pollination demonstrates how a key ecological service can serve as a yardstick for judging restoration success. Key-words: complex system, ecological restoration, food web, heathland, network, pollination web, pollination Introduction Terrestrial restoration projects focus normally on some basic core of the target community, usually the dominant plant *Correspondence author. E-mail: [email protected] †Present address: Department of Ecology, Evolution and Natural species, in the hope that natural processes will subsequently Resources, 1 College Farm Road, Rutgers University, New Brunswick, steer the community on a trajectory towards complete restora- NJ 08901–1582, USA. tion (Palmer, Ambrose & Poff 1997). However, ecologists © 2007 The Authors. Journal compilation © 2007 British Ecological Society Plant–pollinator networks on heathlands 743 rarely possess complete records of the structure of a historical In addition to providing a target community structure, community, let alone regional species pools. This makes it dif- information on interaction networks also enables the robust- ficult to define an exact target community for the restoration ness of a whole community to be ascertained, providing a process and to evaluate later if the community has been powerful tool to test the impact, on community structure, returned to its original state. Moreover, an approach that aims of ‘that most insidious type of extinction, the extinction of to restore some historical community structure is funda- ecological interactions’ (Janzen 1974). For example, model- mentally flawed where landscape properties have changed ling the removal of pollinator species from two plant–pollinator and can no longer sustain the target community (Cairns & networks, Memmott, Waser & Price (2004) found that the Heckman 1996; Palmer, Ambrose & Poff 1997; Ehrenfeld greatest effect on plant survival occurred when generalist 2000). Similarly, the use of reference communities as targets pollinators were removed. By comparing the robustness of to aim for in the restoration process can be inappropriate, networks from restored sites to those from ancient sites, the because species composition is expected to vary between ability of restored sites to withstand future natural and man- localities in response to variations in soil type, hydrology, made perturbations can be assessed. aspect, disturbance frequency, size and composition of the Here we use a network approach to evaluate the outcome of local species pool, landscape connectivity, priority effects and heathland restoration in the county of Dorset, southern chance events (Jordano 1994; Williams et al. 1996; White & England. We focus on the following three questions: (1) has Walker 1997; Potts et al. 2003; Young, Petersen & Clary 2005). pollination been reinstated in restored heathlands; (2) do In planning and evaluating restoration projects, therefore, a insect species which visit flowers on restored heaths originate purely structural focus is inadequate. An alternative is to from adjacent ancient heathland; and (3) do ancient heaths consider function, i.e. what constituent species do rather than have a more complex network structure than restored heaths simply recording whether or not they are present (Ehrenfeld and is any variation in complexity linked to the resilience of & Toth 1997). Indeed, a direct functional comparison of the networks to future perturbation? successfully restored and target habitat is possible when con- sidering ecological processes that should not vary between localities, such as processes needed for target plants to survive Materials and methods and reproduce. Pollination is one of several ecosystem services that must be BRITISH HEATHLANDS reinstated for ecological restoration to be successful. Pollina- Dry lowland heathland is a relatively species-poor, seminatural plant tion by animals is ubiquitous in terrestrial habitats, involving community dominated by the ericaceous shrubs Calluna vulgaris 67% of flowering plant species (Kearns & Inouye 1997) and Hull, Erica tetralix L. and E. cinerea L., and the fabaceous shrubs an equivalently high diversity of insect species (Nabhan & Ulex minor Roth, U. europaeus L. and/or U. gallii Planch. The habitat Buchmann 1997). However, plant–pollinator interactions is an important focus for conservation and ecological restoration may not re-establish automatically themselves in communities efforts, because heathlands have a limited distribution internation- undergoing restoration management, because pollinators ally, are associated with a number of rare or threatened species of establish populations only once their habitat requirements vertebrates and invertebrates (Usher 1992; Anonymous 2002) and have considerable aesthetic and cultural significance (Webb 1986; have been met. For example, in addition to food resources, Usher 1992; Anonymous 2002). The Dorset heathlands were once bees require nesting sites and nesting materials (e.g. Kearns, extensive: a survey in the 1750s listed some 40 000 ha, but by 1978 Inouye & Waser 1998; Gathmann & Tscharntke 2002). These only 6000 ha remained (Webb & Haskins 1980). This reduction was features make pollination a useful functional bio-indicator for caused predominantly by afforestation, conversion to agriculture, comparing restored communities to reference communities. urban spread and a lack of appropriate management to halt succes- Connections between plants and pollinators can be analysed sion (Rose et al. 2000). The heathland that remains is highly frag- for entire ecological communities using a food web approach mented but ecological restoration is currently being carried out to (Jordano 1987; Waser et al. 1996; Memmott 1999; Dicks, Corbet increase the total heathland area and link up heath fragments. & Pywell 2002; Memmott & Waser 2002; Vázquez & Aizen 2003) or, more generally, a complex systems approach (Bascompte THE FIELD SITES et al. 2003; Jordano, Bascompte & Olesen 2003; Memmott, Waser & Price 2004). The recent surge in work investigating We used a paired design to study restored heathlands and compare the properties and consequences of network structure has their vegetation and visitation networks to those of ancient heath- > yielded many new insights (Jordano, Bascompte & Olesen lands ( 250 years old). Thus each of the four restored sites (R1–R4) 2006), some of which have implications for restoration ecology. was paired with an ancient site (A1–A4). Paired sites were adjacent, except pair 2, where the sites were separated by 3 km (although R2 was For example, the way interaction webs assemble is relevant

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